An electrode [ elɛkˈtroːdə ] (from ancient Greek ηλεκτρόν electron , " amber ", i. Ü . S. "electrical", and ὁδός hodós, "path") is an electron conductor that interacts with a counter electrode ( anode - cathode ) with a medium located between the two electrodes interacts. Electrodes consist of electrical conductors, usually a metal or graphite . They are used to connect non-electron-conducting areas with cables and are used, for example, in electrochemical elements, as tools (e.g. for resistance spot welding ) and, if necessary, material dispensers for electric welding , as connections and electron-optical elements in electron tubes . In addition to its electrical function, electrode material can be deposited or consumed, or physical processes can take place in the electrode, such as in the anode of an X-ray tube .
Electrodes in gas or vacuum or with an insulator
Depending on the type of medium surrounding the electrode, there are different forms of interaction:
If the medium is a vacuum or a gas , an electric field builds up between the electrodes , as in the case of the insulator . However, electrons can move from one electrode, the cathode , to the other if the exit from the cathode is allowed, e.g. B. by field emission or thermal emission or the Edison-Richardson effect as is the case with various electron tubes with hot cathodes .
If the medium is a gas, the atoms or molecules of the gas are partially ionized, so that a plasma is created. In the plasma, in addition to the electrons, the ions also move in the electric field, like in a gas discharge lamp .
The electrodes of the spark plugs , the welding electrodes for electric welding and the electrodes inside the nozzle for plasma fusion cutting also fall into this category. When welding, the welding electrodes generate an arc with the material to be welded. In the heat of the arc, both melt and the electrode serves as a filler material, so that the materials are connected.
Here the medium that is adjacent to the electrode is a liquid or solid ion conductor , the electrolyte . An electrochemical potential builds up on the electrode through oxidation and reduction reactions or through an external voltage .
A distinction is made between four types of electrodes, depending on how the potential is dependent on the concentration of the electrolyte:
- First type electrodes
- Electrodes of the second type in which solids are involved in the reaction
- Redox electrodes; here no metal ions, but electrons pass through the phase boundaries. The metal itself is not attacked because there is no mass transport through the phase boundaries.
- Ion-selective electrodes , in which the potential ideally depends on the concentration of only one specific ion.
If you connect two electrodes, which are immersed in electrolyte solutions of different concentrations or made of different materials, to one another via a circuit, you get a galvanic element . A voltage can be measured between the electrodes which results from the potential difference and which is called the electromotive force (EMF) or "reversible cell voltage". Such an arrangement can deliver electricity ( battery ). When an external voltage is switched on, other chemical reactions take place on the electrodes ( electrolysis ). The electrodes can be made of metals or semiconductors , e.g. B. also made of graphite , vitreous carbon, and can be liquid ( mercury ) or solid.
One electrode used for corrosion protection is the sacrificial anode .
Polarity of electrochemical electrodes
The following applies to the electrochemical electrodes:
- The electrode on which the oxidation takes place is the anode . Electrons flow from the anode via a conductor. Anions in solution flow to the anode.
- The electrode on which the reduction takes place is the cathode . Electrons flow to the cathode via a conductor. Cations in solution flow to the cathode.
Which of the two electrodes is positive and which is negative depends on the electrochemical device:
- If the chemical reaction is forced by a current flow caused by an external voltage ( electrolysis , galvanization ), the oxidation is caused by the withdrawal of electrons from the positively charged anode: In this case, the anode is the positive pole (+).
- If the electrical voltage is generated by the chemical processes, such as in galvanic cells ( battery or fuel cell ), the anode is negatively charged because electrons are released during the voluntary oxidation. The anode is then the negative pole (-).
Since the polarity of the electrodes is reversed during electrolysis compared to a galvanic element, the assignment of anode and cathode is often confusing. However, one can orientate oneself on the direction of flow of the electrons. To do this, imagine the cell schematically: The prefix ana- means upwards, the prefix kata- means downwards. As a rule, the anode is shown in the drawings on the left, the cathode on the right.
First type electrodes
Electrodes of the first type are electrodes whose potential depends directly on the concentration of the electrolyte solution surrounding them. These are, for example, all metals that are immersed in a solution of their metal ions (electrolyte solution). At the phase boundary , an equilibrium is established between the solution pressure of the metal and the osmotic pressure of the electrolyte solution.
- The pressure of the metal to dissolve comes about because each metal tries to release cations from its lattice . Due to the excess of electrons in the metal, the metal charges itself negatively. As a result of the Coulomb attraction, the cations remain in relative proximity to the electrode. An electrochemical double layer forms. The ability of a metal to release cations from its lattice was listed for each metal in the electrochemical series . The lower it is, the more ignoble it is and the greater is its ability to release cations.
- The opposite tendency is due to the osmotic pressure of the electrolyte solution or, to put it more simply: electrolyte solutions want to dilute. They achieve this by forcing the dissolved metal ions into the grid of the electrode and installing them there. They do this particularly well when many metal ions are present in dissolved form. This leads to the formation of an electrochemical double layer with the opposite sign. This opposite trend is supported by the electrostatic attraction of the dissolved salt ions by the electrons remaining in the metal during the dissolution process of the metal.
So solution pressure, as well as osmotic pressure and electrical pressure, are in balance. Which side (solution pressure vs. osmotic pressure) the equilibrium lies on depends on the one hand on the position of the metal in the electrochemical series and on the other hand on the concentration of the electrolyte solution.
Electrodes of the second kind
Electrodes of the second type are electrodes whose potential depends only indirectly on the concentration of the electrolyte solution surrounding them . However, the deviation from the electrode of the first type is only a constant voltage difference. Electrodes of the second type are used as reference electrodes .
The concentration independence of the potential is achieved by the special structure of the electrode. More precisely, the special composition of the electrolyte solution keeps the potential constant. The electrolyte solution consists on the one hand of a saturated solution of a sparingly soluble salt , which as a cation consists of the same metal as the electrode and on the other hand of a readily soluble and precisely concentrated alkali salt which contains the same anion as the sparingly soluble salt.
The potential depends on the concentration of the cation of the sparingly soluble salt. This concentration is in turn linked to the concentration of the anion via the solubility product . If the concentration of the anion is kept constant, the potential consequently also remains constant. This anion concentration can be kept almost constant by choosing a very high concentration. By subtracting these voltage values from the measured value, you get the actual potential or the EMF of a solution.
Ion selective electrodes
The potential measured on ion-selective electrodes depends on the concentration (more precisely, activity ) of a certain type of ion. Such an electrode consists in principle of an electrode that does not take part in the electrochemical reaction, e.g. B. a graphite electrode, and an associated electrode phase, which in the simplest case consists of a sparingly soluble salt with the type of ion that is in equilibrium with the corresponding ion in the solution. In practice, ion-selective electrodes are constructed similarly to a glass electrode , the membranes being ion-specific, e.g. B. from the sparingly soluble salt (crystal membrane) or from potash or soda glasses. Liquid membranes consist of an inert carrier on which ionophores dissolved in organic solvents are attached . The selectivity of the electrode is essentially determined by the solubility product and the ionic conductivity . It should also be noted here that many other ions that are present in the solution at the same time can form a disruptive factor.
- In the case of a cadmium-selective electrode, the solid electrode phase or the membrane consists of cadmium sulfide .
- With a silver chloride - silver sulfide electrode, which has a silver chloride-silver sulfide mixed crystal membrane, the concentrations of silver ions as well as those of chloride and sulfide ions can be determined.
A large number of ions are now selectively determined. In analytical practice, u. a. The fluoride electrode is used to determine fluoride ions in water with an accuracy of up to 0.01 mg / l, and in a modified principle, the ammonia electrode (a gas-permeable membrane electrode) is used. A special development are electrochemical biosensors , e.g. B. Enzyme Electrodes.
The advantages of microelectrodes are the very small total current (only slight disturbance of the system to be examined by reaction products, only a small voltage drop in the solution), no influence of (moderate) currents on the measurement result, detectability of very small concentrations, negligible capacitive effects (this enables very high measuring rates), and the feasibility of very high current densities. These are offset by disadvantages such as a small total current despite a high current density and an extremely large ratio of sample volume to electrode surface (even if only traces of surface-active substances are present, they easily cover the entire electrode surface).
The terms electrode, electrolyte, anode and cathode arose at the suggestion of Michael Faraday (1791–1867) and were made public by him. Faraday, who had not learned Greek, was advised by William Whewell (1794–1866), the rector of Trinity College at the University of Cambridge .
- International Electrotechnic Vocabulary (IEV), published by the International Electrotechnic Commission (IEC)
- The IEV on the Internet
- Ion-selective electrodes at the University of Jena (PDF; 184 kB)
- Ion-selective electrodes at the University of Bern ( Memento from December 15, 2011 in the Internet Archive ) (PDF; 50 kB)
- Video: Electrodes (Basic Knowledge of Physical Chemistry # 12) . Jakob Günter Lauth (SciFox) 2019, made available by the Technical Information Library (TIB), doi : 10.5446 / 40359 .
- Charles E. Mortimer, Ulrich Müller: Chemistry. 8th edition, Georg Thieme Verlag KG, 2007, ISBN 978-3-13-484309-5 , p. 352.
- H. Wenck, K. Hörner: Ion-selective electrodes , chemistry in our time, 23rd year 1989, no. 6, p. 207
- Ludwig Pohlmann: Electrochemical measuring methods: microelectrodes
- https://www.plasma.uaic.ro/topala/articole/Faraday%201834%20VII.pdf Section 662, 664 and 663 respectively